Jason Groome is a 6'6" 220 lb Left Handed Pitcher from Barnegat, NJ with a College commitment to Vanderbilt (Groome himself has stated he's 6'5" and 230 lbs, but I use what I find officially listed). The first thing to note is his frame. It's rather insane for a High Schooler. Oftentimes you'll see a kid who grew quickly and still needs to fill out, but Groome already sports a big frame, with some room to grow, but not much. In this case, that's fine as Groome reportedly already hits mid-to-upper 90's with his Fastball, making it an easy plus pitch for a Lefty. Along with the heater, he also sports a Curveball and a Changeup that is pretty advanced for a High Schooler (though to keep perspective, that's still a pitch that will need to progress quite a bit, he's just at a better starting point than his peers).
I already mentioned that Groome has a big Fastball, but there's more to a Fastball than speed. First to note on Groome's Fastball is one of the better deliveries I've seen. He's balanced, toes point to home on his landing foot, his arm slot is consistent on all his pitches and he repeats his delivery very well. This all suggests he should end up with good Command of his pitches. However, at present his command of his Fastball is roughly average. His Control is fine, he can get it over the plate consistently, but not always where he wants it (or at least where the Catcher wants it). The pitch has some sink and occasional arm-side run to it, but not to the degree that his Command isn't going to come later. I would imagine this will top out as a Plus-Plus pitch with Above-Average command and like most Fastballs, the command will be better when thrown lower in his upper range. His pro team will want to work on his consistency in the movement of the pitch, as that can vary quite a bit right now.
Next up is his Changeup, a pitch that you don't see much of, frankly. The best way to look at this is with an analogy to Tecmo Bowl (for those of us in the 40-ish range. Kids, ask your parents or dead-beat uncles). When you played that game you just ran the Sweep every time. Sure, you could go with a Dive play or perhaps a pass, but the Sweep was unstoppable. It's like that for most High School Pitchers good enough to be in First Round pick discussions. They generally have Fastballs that other High Schoolers can't catch up with, so the Pitchers often just throw a near steady diet of Fastballs. For example, in Groome's first game this year he was on a 60-pitch count. of those he threw 14 Curveballs, 2 Changeups and everything else was all Fastballs. The general review of the Changeups that writers and scouts have seen is that they are Average and the pitch is expected to remain that level. The pitch comes in ~10 mph slower than the Fastball with some fade and is released from the same arm angle and arm speed as the Fastball. It's a safe bet that whomever takes Groome this year will send him to A ball with strict commands to throw an absurd volume of Changeups in the hopes the pitch can be refined and, ehem, groomed into an Above-Average offering.
I have saved the best pitch of this draft class for last. I LOVE Groome's Curveball. Technically it's a Knuckle Curve, and it's pretty awesome. I like the variability he can bring to it (though scouts are less fond of that). What a Knuckle Curve does is that the Index Finger pinches the ball in the same way a Knuckleballer pinches with all his fingertips. This adds the tight spin for a late break, then moving the other fingers closer or further from the pinch can alter the shape turning it to more of a Slider or making it a bit loopy. When the Grip is on, this pitch looks easy Plus and some have rated it as high as a 70 on the 20-80 scale. The problem is that the consistency isn't quite there and the pitch can get pretty loopy. A tweaking of the grip to be more consistent is the likely fix.
Between those 3 pitches alone lies #1 Starter potential. A lot of comparisons have been to Clayton Kershaw, but I see Madison Bumgarner as a better comp. The builds are similar, but the stuff and projected command is closer to where Mad Bum was. That's still a damned good Pitcher. Working against Groome is that he's a High Schooler, which is a fairly risky profile, and some concerns about asking price. The asking price is key because 1) his agent is
Scott Boras Jeff Randazo (h/t ReadingPhilly)who will ask for top dollar (as is his job) and 2) Vanderbilt does have a reputation for losing fewer Pitchers than other schools.
It's important to remember that while all of the parts here suggest a possible Ace, there are just enough flaws with each that there is some risk present. If the Pitches don't progress with pro-coaching he would likely max out as a rather frustrating #3/4 Pitcher who shows flashes of his potential. Recent rumors have the Phillies leaning towards a College player. However, the Phillies have fairly heavily scouted Groome with Klentak watching him early this season in person. Groome could still be the pick, and he's been my preference for a while. That said, the Phillies have not picked my preferred guy 3 years running and it's going quite well (let's not talk about my preferred players, thank you), so I'll allow some trust in their process. Pro teams have far more information than I do (or any prospect writer, frankly) and unlike me, they're trained to do this work. Most years Groome would be a likely Top 5 pick, but probably only loosely mentioned for 1:1, this isn't a very star laden Draft though so he could go anywhere from 1-10, though I think it's unlikely he goes beyond 5.
I'll leave you with the video below. As I said above, I quite like his mechanics so no critiques on it this time. (Video courtesy of Baseball America)